Annular illumination decreases the input resistance of turtle cones and results in a depolarizing response when the intracellular concentration of C1- is raised. Hyperpolarizing current blocks this synaptic response, while depolarizing pulses in darkness evoke potential and resistance changes similar to those associated with the surround effect. Furthermore, the response to annular illumination is transient and shows refractoriness. From these observations, it is suggested that the surround response of cones arises from the activation of a voltage- and time-dependent C1- conductance.